Liquid Fuel Burners

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Liquid Fuel Burners

• Oils may be burnt in two ways
• it is vaporized before ignition so that it burns
  like a gas
• (vaporising burners)
• it is converted into droplets which are injected
  into hot air so that they evaporate while burning
  (atomising burners)

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Liquid Fuel Burners

• Atomising burners On industrial scale, most
  commonly used burners are atomising burners
• Oil is heated to low viscosity and atomised
• Mechanically by means of a rotating disc or cup
  with a uniform droplet size (50 microns)
• By a high pressure ejection from a fine orifice
  which gives a conical spray

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Types of Atomising Oil Burners

• There are three types which differ on the
  principal of atomising
• Pressure Jet Atomising Burners
• Blast Atomising Burners
• Rotary Atomising Burners

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Pressure Jet Atomising Burners

• Oil enters the circular swirl chamber through
  tangentially spaced slots
• Oil will rotate in the chamber around the air
• The rotating mass is passed through an orifice
  resulting in the formation of spray of drops
• The viscosity should be 70 Redwood I seconds for
  small nozzles and 100 Redwood I seconds for large
  nozzles

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• These burners have low operating cost and most
  widely used
• These type of burners have limited turndown ratio
• Turndown ratio can be increased by design
  modifications e.g. by increasing the number of
  tangential slots

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Blast Atomising Burners

• These burners use air or steam to atomize the oil
• Oil flow through a central tube at a controlled
  rate mixed with mixed with air as it emerges from
  the tube
• Depending upon the pressure they may be
  classified as Low pressure, Medium pressure or
  high pressure
• Depending upon the mixing system they may be
  classified as inside-mix type or outside-mix type

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Blast Atomising Burners

• High pressure burners have high turndown ratio
  (101)
• Inside-mix type commonly provide more eficiency

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Blast Atomising Burners
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Rotary Atomising Burners

• These burners have a centeral stationary fuel
  line which delivers the oil to the inner surface
  of rotating hollow cup
• The cup is rotated at 3600- 10,000 rpm
• Centrifugal force causes the oil to flow towards
  the brim of the cup in the form of thin film
  which disintegrates into small droplets
• A fan attached to the rotating shaft provides
  primary air
• These burners can be used for more viscous fuels
• Low viscosity may cause the oil to slip within
  the cup resulting in low atomizing efficiency
• These burners can have high turn down ratio but
  low capacity

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Rotary Atomising Burners
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Adiabatic Flame Temperature

• For adiabatic flame temperature following
  assumptions are made
• No heat loss to the surroundings
• Combustion is complete
• No thermal dissociation
• A reference/datum temperature is selected

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Adiabatic Flame Temperature
Fuel
Adiabatic flame
Combustion products
oxident
diluent
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Adiabatic Flame Temperature

• A fuel gas containing 20 CO and 80 N2 is
  burned with 150 excess air (both air and gas
  being at 25 C). Calculate the theoretical flame
  temperaure of the gas.
• Following data is available

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CO2 O2 N2 CO
Av. Sp. Heat kcal/mole K 12.10 7.90 7.55 -
Heat of formation at 25 C kcal/kg mole -94052 -26412
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Material BalanceBasis 100 kg moles of fuel gas
Material entering Kg moles Materials leaving Kg moles
CO N2 O2 CO2 20 80 174.05 25 - - 80 174.05 15 20
total 299.05 289.05
Energy Balance assuming reference temp. 25
C Heat of reaction -94052 (-26412) -67640
kcal / kg mole Heat produced by combustion 20 x
-67640 -1352800
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Cp kcal / kg mole K Amount kg mole mCp dT
CO2 12.10 20 20x12.10xdT
O2 7.90 15 7.90 x 15 x dT
N2 7.55 174.05 7.55 x 174.05 x dT
Total1674.58 dT
1674.58(Tf -298) 1352800 Tf 832.8 C
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Adiabatic Flame Temperature

• NCV?hf A?HaV?Hfg qd ql
• A air supplied m3/m3 fuel
• Vflue gases produced m3/m3 of fuel
• ?Hfenthalpy of fuel above reference temperature
• ?Hfgtf. Cpfg(0-tf) trCpfg(0-tr)
• Tf(NCV?hf A?Ha -qd ql V. trCpfg(0-tr))/ V.
  Cpfg(0-tf)

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Adiabatic Flame Temperature

• Calculate the theoretical flame temperature for a
  fuel gas under the following conditions
• Both fuel and theoretical air are at 15 C
• 50 excess air at 15 C and fuel gas at 15 C
• Theoretical air at 60 C and gas at 400 C
• Theoretical oxygen at 15 C and fuel gas at 15 C
• Data
• Fuel gas CO 22 CO2 18 H2 2 N2 58
• NCV 719 kcal/m3
• Mean sp. Heat of fuel gas at 600 C 0.342 kcal/m3
  C